Various instruments have found use for applications requiring level sensing of, for example, a liquid or bulk material in a vessel. One such instrument comprises a time domain reflectometry measurement instrument. With any such instrument it is desirable to ensure safety of both equipment and personnel.
A time domain reflectometry measurement instrument uses guided wave radar for sensing level. The instrument includes a control housing and a probe. The probe is typically mounted to a process vessel using a threaded fitting or a flange fitting. The probe comprises a high frequency transmission line which, when placed in the vessel, is used to measure level of fluid in the vessel. The probe is controlled by electronics in the control housing for determining level in the vessel.
Process instruments are sometimes used for sensing level of a pressurized vessel. Often, the contents of the vessel is at a high temperature. In order to seal the vessel at the probe connection, an appropriate seal must be included. The seal must be appropriate for the relatively high temperatures and pressures when used in such applications.
A time domain reflectometry instrument, as well as certain other process instruments, imparts a pulse signal on the probe and receives a return signal. The relationship between the pulse signal and the return signal is used to determine level. Typically, the control housing includes a 50 ohm control connection for the probe. If the probe has a different impedance, then a mismatch results. The mismatch can cause errors in level measurement. In applications where the probe has a 50 ohm impedance process connection, matching that of the control electrical connection, the design of the process seal can cause an impedance mismatch.
The present invention is intended to satisfy one or more of the problems discussed above.